The viability of precision-cut rat liver slices has been assessed in two different culture systems. Liver slices cultured in sealed vessels gassed with 5% carbon dioxide and 95% oxygen only remained viable for about 12 hr when judged by potassium content but 18 hr when judged by enzyme leakage. However, rat liver slices remained viable for up to 24 hr as judged by both these criteria when unsealed culture vessels were placed in a humidified atmosphere of 5% carbon dioxide in air. The effect of menadione (2-methyl-1,4-naphthoquinone) on the viability of slices and hepatocytes in culture was compared. The results indicate that signs of toxicity may be observed in cultured hepatocytes within 1-2 hr in medium containing 200 mum-menadione. In contrast, no signs of toxicity were observed in slices cultured in medium containing 200 mum-menadione for at least 12 hr. Higher concentrations of menadione did give rise to toxicity in cultured slices. Evidence that preserved levels of glycogen in liver slices may in part underlie their resistance to menadione-induced toxicity is presented. These results suggest that precision-cut liver slices may more accurately reflect the hepatotoxic potential of chemicals in short-term tests than do isolated hepatocytes.